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Phagosomal processing of Mycobacterium tuberculosis antigen 85B is modulated independently of mycobacterial viability

Lakshmi Ramachandra1, Jamie L Smialek, Sam S Shank

  • 1Department of Pediatrics, Case Western Reserve University and Rainbow Babies and Children's Hospital, Room 4007, 11100 Euclid Avenue, Cleveland, OH 44106-6008B, USA. lxr2@cwru.edu

Infection and Immunity
|January 25, 2005
PubMed

Insights

Heat-killed Mycobacterium tuberculosis (M. tuberculosis) is processed more efficiently by macrophages than live or killed bacteria. This difference in antigen processing is not due to phagosome maturation but suggests distinct bacterial mechanisms.

Area of Science:

  • Immunology
  • Microbiology
  • Cell Biology

Background:

  • Control of Mycobacterium tuberculosis infection relies on CD4 T-cell responses and major histocompatibility complex class II (MHC-II) antigen processing.
  • Macrophages process heat-killed (HK) M. tuberculosis more efficiently than live M. tuberculosis, suggesting potential inhibition of antigen processing or altered accessibility.

Purpose of the Study:

  • To investigate the correlation between M. tuberculosis viability and phagosome maturation, and its impact on antigen processing efficiency.
  • To determine if bacterial killing method (heat, radiation, antibiotics) affects antigen processing and presentation.

Main Methods:

  • Activated murine bone marrow-derived macrophages were used to process heat-killed (HK), live, radiation-killed (RadK), or rifampin-killed (RifK) M. tuberculosis.
  • Processing efficiency was assessed using an I-A(b)-restricted T-cell hybridoma (BB7) recognizing an Ag 85B epitope.
  • Phagosome maturation markers (acidification) and peptide-MHC-II complex expression were analyzed.

Main Results:

  • Macrophages processed HK M. tuberculosis significantly more rapidly and efficiently than live, RadK, or RifK M. tuberculosis.
  • Live, RadK, and RifK M. tuberculosis were processed with similar, lower efficiencies for T-cell presentation.
  • Phagosomes containing HK M. tuberculosis showed higher expression of M. tuberculosis peptide-MHC-II complexes compared to those with live or RadK bacteria.
  • Only live M. tuberculosis inhibited phagosome acidification, suggesting phagosome maturation regulation is not the primary factor in processing differences.

Conclusions:

  • Bacterial viability and preparation method (e.g., heat-killing) critically influence M. tuberculosis antigen processing by macrophages.
  • The observed differences in antigen processing efficiency are not explained by the modulation of phagosome maturation.
  • M. tuberculosis employs distinct mechanisms to inhibit phagosomal maturation versus modulating phagosome antigen processing.

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